Wildly fluctuating engine tenperature 98 Sierra 1500

Tiribulus · April 22, 2023

Hi folks,

I'm hoping somebody can tell me what's going on here. I know just enough about cars to get myself in trouble.

I have a 1998 Sierra 1500 pickup, 190,000 miles, Vortex 5.7 2WD.

It runs fine overall, but over the last several months it has begun doing this thing where the engine temperature will jump back forth between normal temp and WAY up. I can watch it move up and down on the instrument panel. It doesn't do it constantly, but regularly. Several seconds climbing up, it'll stay up for a couple minutes and then drop back down over several seconds and so on.

I changed the thermostat and filled up the coolant a couple months ago and it seemed ok for a while, but is doing it again now.

This morning I had some smoke or steam coming out from under the hood. Not a ton, but it was. I took a look at it, but can't tell if it's gunk burning off the block and valve cover or steam escaping from somewhere. It smells more like steam and it's only coming from the drivers side of the motor toward the front.

Some possible clues that may or may not be related in various degrees.

When I start it sometimes there is a distinct ticking. Sometimes. At first I thought it was a lifter, but I've never heard of a ticking lifter that stops ticking intermittently. The ticking, when present, stops after a few minutes of the engine running. It Seems that whenever it starts up with the immediate ticking the temperature issue isn't as bad, but when it starts without the ticking, the temp goes up faster and the fluctuation is worse. That SEEMS to be a pattern. I can't confirm that 100%.

After a couple minutes of running I hear the sloshing of water running through something under the dashboard (heater core?) Also the cab heat seems to be working less and less and when the engine temp is up it doesn't work at all.

The main top radiator hose has very little pressure in it. Either when running or after I turn it off. I can easily squeeze it either way.

It is going through coolant with fair regularity. I have to add like a half gallon like once a week so maybe this is just a leak somewhere?

Money is really tight. I can't afford to take it somewhere right now.

I know it's tough to diagnose anything in a forum thread without seeing it, but any insight on this would be very much appreciated.

Thanks so much.

silveradosid · April 23, 2023

you have a head gasket problem or even possible cracked head. your temp gauge and heater core are telling you you have air in the system. i cant remember what years it involved but gm did have a problem of head cracking. you could try a stop leak product, just make sure to pinch off the heater lines if you do use a stop leak product

Tiribulus · April 23, 2023

Ok, I put in some Bar's Leaks copper according to the instructions and that stopped the leak. Or certainly seems to have stopped it. No more spittling or steam.

Here's a REALLY good one now though that I don't think is related.

Last week, the truck wouldn't start. After driving it to the store, in the parking lot. Full battery power near as I could tell, but no click, nuthin. Then on like the 5th or 6th try it started right up like normal and has ever since. (starter or open circuit?)

Just now, I started it up and it was making a buzzing or whirring noise seemingly in the vicinity of the starter. I turned it off and the noise continued after it was off and now it won't start again. Just like last week.

Lights, windows everything fine, but no response from the ignition and it makes that whirring/buzzing noise while it's not even running. I had to disconnect the battery to get it to stop. Never did anything like this before.

This is a brand new one on me. I have no idea what this is. If anybody else does, you'll be my new hero. 195,000 miles on this truck btw ,which runs fine overall until these recent problems.
Thanks again.

Edited April 23, 2023 by Tiribulus

Tiribulus · April 23, 2023

I went back out there just now and reconnected the battery terminal and as soon as the circuit is closed, the electrical motor whine/buzz sound started up until until I broke the circuit again.

Mike Duhaime · April 23, 2023

Hi

first off you're in the wrong forum....there is one for older trucks that people can help better

the issues you have are different than newer trucks. At that vintage and age, corrosion is big issue.

But, i'm an auto , truck and construction tech. A temp gauge that goes up and down fast may be a false message.

You need a laser temp gun. if the truck is really overheating, you will see over 220 degrees. Many times, a bad wire affects the temp gauge. Usually at plugs or harness connections. If you are overheating, there is gurgling in rad, overflow fills up, snapping noises. You need to know with a simple laser heat gun (good to check kids temp, food too!)

you may have a shorted solenoid on starter. You must unhook cable at starter to test. You may have bad battery, bad connections at ground. At this point, there are electrical issues that you need a mechanic to test. Good luck